Conventionally, sheet feeders have had a separating function for separating and feeding a plurality of sheets in order to feed the sheets one at a time in a feeding direction. For example, one of such conventional sheet feeders includes a feed roller and a retard roller arranged in confrontation with each other. To activate the separating function of the sheet feeder, the retard roller is rotated in a direction opposite the feeding direction, while the feed roller is rotated in the feeding direction.